The present invention, in some embodiments thereof, relates to the manipulation of a fluidic medium and, more particularly, but not exclusively, to the manipulation of a fluidic medium by light.
Manipulation of liquid, particularly in microchannels, has to attracted research and industrial attention for many years. For example, U.S. Pat. No. 6,294,063 to Becker et al. describes microfluidic devices that manipulate packets of fluids through the application of electric fields via electrodes located on the devices. A fluid is introduced onto a reaction surface and compartmentalized to form a packet. An adjustable programmable manipulation force is applied to the packet according to the position of the packet. As a result, the packet is moved according to the manipulation force. In some cases, electromagnetic radiation may be used to maintain photochemical reaction or for sensing processes.
Also known are techniques which rely on the ability of electric fields to change the contact angle of a fluid on a surface. For example, application of electric field gradient to a droplet on a fluid-transporting surface, to form different contact angles between leading and receding surfaces of the droplet with respect to the fluid-transporting surface, hence to cause imbalance in surface tension forces which produces a net force and move the droplet [Lee et al. (2002), “Electrowetting and electrowetting-on-dielectric for microscale liquid handling,” Sensors and Actuators A 95:259-268]. Another example is the use of a particular surface polymer layer which changes its isomeric form once being exposed to ultraviolet or blue light. This change of isomeric form changes the contact angle between the polymer layer and a macroscopic droplet placed thereon [Ichimura et al. (2000), “Light-driven motion of liquids on a photoresponsive surface,” Science 288:1624-1626].
Optical trapping is a phenomenon in which items such as atoms, molecules and small particles are manipulated by light. The fundamental principle behind optical trapping is that light carries momentum, which can then be expressed as radiation pressure. When light is absorbed, reflected or refracted by a material, momentum is transferred to the material.
One type of optical trap is a single-beam gradient trap, also known as “optical tweezers”. A laser beam is focused on the particles which are typically in a liquid medium on a microscope slide.
U.S. Pat. Nos. 3,710,279 and 3,808,550 to Askin disclose a variety apparatus for controlling by radiation pressure the motion of particle, such as a neutral biological particle, free to move with respect to its environment. U.S. Pat. No. 4,893,886 to Ashkin, et al. discloses biological particles such as cells, bacteria, and viruses, which are successfully trapped in a single-beam gradient force trap using an infrared laser. The high numerical aperture lens objective in the trap is also used for simultaneous viewing. Other exemplary applications of optical tweezers technology include the immobilization of biomolecules such as DNA, RNA, proteins, lipids, carbohydrates, or hormones (see U.S. Pat. No. 6,139,831 to Shivashankar et al.).
U.S. Pat. No. 6,995,351 to Curtis et al. describes an implementation of dynamic holographic optical tweezers for producing many independent traps. Computer-generated diffractive optical elements are used for converting a single beam into multiple traps which in turn are used to form one or more optical vortices. The optical vortex technique is combined with the holographic optical tweezers technique to create multiple optical vortices in arbitrary configurations. The rotation induced in trapped particles by optical vortices is employed for forming an array of co-rotating rings of particles. The flow outside a ring of particles is used to pump fluids through small channels.